Actuatable passenger restraint systems for vehicles are well known in the art. One particular type of actuatable passenger restraint system includes an inflatable air bag mounted within the passenger compartment of the vehicle. Each air bag in the vehicle has an associated, electrically actuatable ignitor, referred to as a squib. Such systems further include an inertia sensing device for measuring the deceleration of the vehicle. When the inertia sensing device indicates that the vehicle is decelerating at a rate above a predetermined amount, an electric current of sufficient magnitude and duration is passed through the squib to ignite the squib which, in turn, ignites a combustible gas generating composition or pierces a container of pressurized gas, thereby inflating the air bag.
Many known inertia sensing devices used in actuatable passenger restraint systems are mechanical in nature. Such devices are typically mounted to the vehicle frame and include a pair of mechanically actuatable switch contacts and a resiliently biased weight. The weight is arranged such that when the vehicle is decelerated, the weight physically moves relative to its mounting. The greater the rate of deceleration, the further the weight moves against the bias force. The switch contacts are mounted relative to the biased weight such that, when the weight moves a predetermined distance, the weight moves over or against the switch contacts causing them to close. The switch contacts, when closed, connect a squib to a source of electrical energy sufficient to ignite the squib.
Still other known actuatable passenger restraint systems for vehicles include an electrical transducer or accelerometer for sensing vehicle deceleration. Such systems include a monitoring or evaluation circuit connected to the output of the transducer. Such transducers provided an electric signal having a value indicative of the vehicle's rate of deceleration. The monitoring circuit processes the transducer output signal. One typical processing technique is to integrate the transducer output signal. If the output of the integrator exceeds a predetermined value, an electrical switch is actuated to connect electrical energy to the squib. One example of such a system is disclosed in Brede, et al. U.S. Pat. No. 3,870,894 ("the '894 patent").
The '894 patent discloses a system which includes an accelerometer, an evaluation circuit connected to the accelerometer, and an ignition circuit or squib connected to an output of the evaluation circuit. The accelerometer includes a piezoelectric transducer that provides an electrical output signal having a value indicative of the vehicle deceleration. The evaluation circuit includes an integrator electrically coupled to the output of the accelerometer through an amplifier. The output of the integrator is an electrical signal having a value indicative of the integral of the deceleration signal. A trigger circuit is connected to the output of the integrator. When the output of the integrator reaches a predetermined value, the trigger circuit actuates a time delay circuit. The time delay circuit begins to time out a predetermined time period. After the time period is timed out, the air bag ignitor is energized.
It has been discovered that it is not desirable to inflate a vehicle air bag under all types of crashes to which the vehicle is subjected. It is not desirable, for example, to inflate the air bag during a low speed, "soft crash." The determination as to what occurrences fall within the definition of "soft crash" is dependent upon various factors related to the type of vehicle. If, for example, a large vehicle traveling eight miles per hour hits a parked vehicle, such a crash would be considered a "soft crash" that would not require the air bag to inflate to protect the vehicle passengers. The vehicle seat belts alone would be sufficient to provide passenger safety. During such a "soft crash," a typical accelerometer would provide an output signal indicating a rapid deceleration is occurring. In an actuatable passenger restraint system made in accordance with the '894 patent, the air bag would be inflated as soon as the predetermined speed differential occurred and the time delay circuit timed out.
Another type of electronic control arrangement for an actuatable passenger restraint system is disclosed in U.S. Pat. No. 4,842,301. The '301 patent discloses an air bag actuation circuit that monitors the acoustic emissions produced during crushing of a vehicle of a type having a welded, unit body structure with a pair of frame side rails extending longitudinally from the front of the vehicle to the back of the vehicle. Two acoustic vibration sensors, in accordance with the '301 patent, are secured as close as possible to the front of respective side rails. The output of each of the sensors is connected to a band pass filter with a frequency range of 200 KHz to 300 KHz so as to exclude lower frequency components. The output of the bandpass filters are connected to envelope detectors. The output of the envelope detectors are connected to comparators. Once the level of the acoustic vibrations in the pass band frequency exceed a value set by the comparator reference, the air bag is actuated.